The present invention relates to a circuit for calculating a DC value applied to a code conversion control apparatus for use in a channel coding method of a digital recording and reproducing system.
Generally speaking, a digital recording and reproducing system should solve the following problems that are basically caused by a video head and tape, and a rotary transformer. 1) Since the rotary transformer cannot transmit, or pass a DC signal, the DC component of the digital signal, that is, the DC levels which represent binary "1" and "0" logic conditions are cut off. This means that the digital signal is recorded without its original DC component. 2) If the video head and the tape are operated in a high frequency area which is over a predetermined frequency, the digital signal cannot be recorded and reproduced by various losses. Accordingly, the minimum run length value T.sub.min should be large. 3) Since a magnetic recording and reproducing system has the characteristic of 6 dB/oct with respect to a low frequency area, a signal having a low frequency component cannot be recorded. Accordingly, the maximum run length value T.sub.min should be small. 4) The frequency bandwidth of a signal which is recorded and reproduced, according to 2) and 3) above, should be narrowed within a predetermined frequency bandwidth. Accordingly, the ratio (T.sub.max /T.sub.min of the maximum run length vs. the minimum run length should be small. 5) During the reproducing period, the variation of DC levels should be suppressed for the benefit of the equalizer's design and the prevention of non-linear distortion. i.e., prevention of deviation from the linear region of a hysteresis curve.